Microwave-Assisted Polymer Synthesis: Recent Developments in a ...

Microwave-Assisted Polymer Synthesis: Recent Developments in ...
Scheme 6. Grafting procedure that was used for the preparation of chitosan-graft-poly(e-caprolactone).
Figure 3. Molecular weight versus time evolution at different
microwave powers for the stannous octanoate catalyzed
ring-opening polymerization of D,L-lactide. Reprinted with permission
from ref. [56]
wave irradiation (domestic microwave oven). It was found
that, up to 255 W, the molecular weight of the polymer
increasedandreachedamaximumwhen90% conversion
was reached (Figure 3). When a higher dose of microwave
energy was applied, the molecular weight first increased
and subsequently decreased in time due to transesterification
reactions. Moreover, the monomer conversion eventually
decreased on applying 510 W of power indicating
depolymerization. These results clearly demonstrate the
effect of microwave power on the ring-opening polymerization.
However, due to the absence of a direct comparison
with thermal heating it is not clear whether this is a thermal
effect or a non-thermal microwave effect.
Similar observations were made by Wang et al. for
the microwave-assisted ring-opening polymerization of
p-dioxanone using a domestic microwave oven. [57] It was
found that the yield and the molecular weight of the
polymers go through a maximum when the microwave
power is increased or when the reaction time at a given
microwave power is increased due to decomposition of the
polymers. Jin et al. studied the formation of b-tricalcium
phosphate/poly(L-lactide-stat-glycolide) composites in a
domestic microwave oven. [58] The b-tricalcium phosphate
was dispersed in a melt of the two monomers and
stannous octanoate and the polymerization was performed
by applying 100 W microwave power. Up to
10 wt.-% of the inorganic material, the molecular weight of
the resulting polymers decreased, but the addition of
more b-tricalcium phosphate resulted in higher molecular
weights, which was attributed to superheating of the salt
under microwave irradiation. The mechanical properties
of the prepared composites were proportional to the
molecular weight of the copolymers and not to the
amount of dispersed particles. Nagahata et al. have
successfully explored a novel route for the synthesis of
poly[(ethylene terephthalate)-stat-isophthalate] starting
from an ethylene isophthalate cyclic dimer and bis(2-
hydroxyethyl) terephthalate under microwave irradiation.
[59] However, the influence of microwave heating on
this polymerization was not addressed in the current
study.
Macromol. Rapid Commun. 2007, 28, 368–386
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 377